Add VS2013, Windows 8.1, Windows Phone 8.0, Windows Phone 8.1 environment support

VS2013 and Windows 8.1 SDK will be selected if the compiler environment for it is loaded and defined options select them otherwise it will fallback to VS2012 or Windows 8.0 SDK.  Since the VS2013 or VS2012 compiler environment is selected before invoking cmake, if both are installed, both can be built separately automated through %COMSPEC% /c of a [name].cmd file containing the environment and build settings.

Currently, VS2013 compiler is using VS2012 libraries with Windows 8.0 SDK. The problem with the change is when the current behavior of using VS2012 or Windows 8.0 SDK is desired. I will test later but I think the build will be broken for VS2013 with this change unless the code is updated to fix any library incompatibility. Either way the new options for ENABLE_VS2013 and ENABLE_WIN81 are here to preserve the current behavior yet encourage and make possible the ability to use the later libraries.  I hope my strategy of adding 2 ENABLE variables is in the spirit of the cmake part of the project.  The transition to VS2013 libraries and Windows 8.1 SDK can thus begin.  For source code, using conditional compilation with _MSC_VER is the right strategy (1800 for VS2013 vs 1700 VS2012).